Radiographic imaging such as x-ray imaging has been used for years in medical applications and for non-destructive testing.
Normally, an x-ray imaging system includes an x-ray source and an x-ray detector system. The x-ray source emits x-rays, which pass through a subject or object to be imaged and are then registered by the x-ray detector system. Since some materials absorb a larger fraction of the x-rays than others, an image is formed of the subject or object.
A challenge for x-ray imaging detectors is to extract maximum information from the detected x-rays to provide input to an image of an object or subject where the object or subject is depicted in terms of density, composition and structure. It is still common to use film-screen as detector but mostly the detectors today provide a digital image.
Modern x-ray detectors normally need to convert the incident x-rays into electrons, this typically takes place through the photoelectric effect or through Compton interaction and the resulting electron are usually creating secondary visible light until its energy is lost and this light is in turn detected by a photo-sensitive material. There are also detectors, which are based on semiconductors and in this case the electrons created by the x-ray are creating electric charge in terms of electron-hole pairs which are collected through an applied electric field.
There are detectors operating in an integrating mode in the sense that they provide an integrated signal from a multitude of x-rays and the signal is only later digitized to retrieve a best guess of the number of incident x-rays in a pixel.
Photon counting detectors have also emerged as a feasible alternative in some applications; currently those detectors are commercially available mainly in mammography. The photon counting detectors have an advantage since in principal the energy for each x-ray can be measured which yields additional information about the composition of the object. This information can be used to increase the image quality and/or to decrease the radiation dose.
When using simple semiconductor materials, as Silicon or Germanium, Compton scattering causes many x-ray photons to convert from a high energy to a low energy before conversion to electron-hole pairs in the detector. This results in a large fraction of the x-ray photons, originally at a higher energy, producing much less electron-hole pairs than expected, which in turn results in a substantial part of the photon flux appearing at the low end of the energy distribution. In order to detect as many of the x-ray photons as possible, it is therefore necessary to detect as low energies as possible.
FIG. 1 is a schematic diagram illustrating examples of the energy spectrum for three different x-ray tube voltages. The energy spectrum is built up by deposited energies from a mix of different types of interactions, including Compton events at the lower energy range and photoelectric absorption events at the higher energy range.
A conventional mechanism to detect x-ray photons through a direct semiconductor detector basically works as follows. The x-ray photons, including also photons after Compton scattering, are converted to electron-hole pairs inside the semiconductor detector, where the number of electron-hole pairs is generally proportional to the photon energy. The electrons and holes are then drifting towards the detector electrodes, then leaving the detector. During this drift, the electrons and holes induce an electrical current in the electrode, a current which may be measured, e.g. through a Charge Sensitive Amplifier (CSA), followed by a Shaping Filter (SF), as schematically illustrated in FIG. 2.
As the number of electrons and holes from one x-ray event is proportional to the x-ray energy, the total charge in one induced current pulse is proportional to this energy. The current pulse is amplified in the Charge Sensitive Amplifier and then filtered by the Shaping Filter. By choosing an appropriate shaping time of the Shaping Filter, the pulse amplitude after filtering is proportional to the total charge in the current pulse, and therefore proportional to the x-ray energy. Following the Shaping Filter, the pulse amplitude is measured by comparing its value with one or several threshold values (Thr) in one or more comparators (COMP), and counters are introduced by which the number of cases when a pulse is larger than the threshold value may be recorded. In this way it is possible to count and/or record the number of X-ray photons with an energy exceeding an energy corresponding to respective threshold value (Thr) which has been detected within a certain time frame.
An inherent problem in any Charge Sensitive Amplifier is that it will add electronic noise to the detected current. In order to avoid detecting noise instead of real x-ray photons, it is therefore important to set the lowest threshold value (Thr) high enough so that the number of times the noise value exceeds the threshold value is low enough not to disturb the detection of x-ray photons. The Shaping Filter has the general property that large values of the shaping time will lead to a long pulse caused by the x-ray photon and reduce the noise amplitude after the filter. Small values of the shaping time will lead to a short pulse and a larger noise amplitude. Therefore, in order to count as many x-ray photons as possible, a large shaping time is desired to minimize noise and allowing the use of a relatively small threshold level.
Another problem in any counting x-ray photon detector is the so called pile-up problem. When the flux rate of x-ray photons is high there may be problems in distinguishing between two subsequent charge pulses. As mentioned above, the pulse length after the filter depends on the shaping time. If this pulse length is larger than the time between two x-ray photon induced charge pulses, the pulses will grow together and the two photons are not distinguishable and may be counted as one pulse. This is called pile-up. The only way to avoid pile-up at high photon flux is thus to use a small shaping time.
In conclusion, there is an inherent conflict here; in order to manage noise a large shaping time is needed and in order to manage pile-up a small shaping time is needed.
In practice a compromise value of the shaping time is normally selected, which is neither optimal for low flux nor for high flux. For low flux, the total number of counted X-ray photons will be too low, because we need to choose a too high threshold value, in order to avoid noise induced counts. For high flux, the total number of counted X-ray photons will also be too low, because of the pile-up effect.
U.S. Pat. No. 7,149,278 and U.S. Pat. No. 7,330,527 disclose a method and system for dynamically controlling the shaping time of a photon-counting energy-sensitive radiation detector to accommodate variations in incident flux levels. The system includes a photon counting channel connected to receive signals from a detector element to provide photon counts as output according to a dynamically variable shaping time. The photon counting channel has to be controlled by a separate controller that includes at least a shaping time controller for controlling the variable shaping time in near real-time based on the photon count output data.
Although this provides a possible remedy to the above-mentioned conflict, the solution proposed in U.S. Pat. No. 7,149,278 and U.S. Pat. No. 7,330,527 requires a dynamically controllable high-performance shaping filter and a separate controller therefore with challenging programming for real-time requirements.
U.S. Pat. No. 5,873,054 relates to a method and apparatus for combinatorial logic signal processor in a digitally based high speed x-ray spectrometer, but does not provide a solution, nor relates to the above-mentioned problem.
U.S. Pat. No. 8,378,310 discloses a reset mechanism for at least partially solving the pile-up problem discussed above.
U.S. Pat. No. 9,482,764 relates to a radiation detector system comprising a semiconductor detector having a surface, and plural pixelated anodes disposed on the surface, at least one of the pixelated anodes configured to generate a collected charge signal corresponding to a charge collected by the pixelated anode and to generate a non-collected charge signal corresponding to a charge collected by an adjacent anode to the pixelated anode. The idea k to determine a collected value for the collected charge signal in the pixelated anode, determine a non-collected value for the non-collected charge signal in the pixelated anode corresponding to the charge collected by the adjacent anode, determine a calibrated value for the charge collected by the adjacent anode using the value for the non-collected charge signal adjusted by a calibration factor, and determine a total charge produced by a charge sharing event collected by the pixelated anode and the adjacent anode using the collected value and the calibrated value. The charge sharing event is counted as a single event related to one of the pixelated anode or the adjacent anode if the total charge of the charge sharing event determined using the collected value and the calibrated value exceeds a predetermined value. A combined value corresponding to a sum of the collected charge signal and the non-collected charge signal is determined, and the non-collected value is determined using a difference between the combined value and the collected value. Two different shapers may be used, a first shaper used for generating a first shaped signal and determining the collected value using the first shaped signal, a second shaper (having a higher frequency than the first shaper) used for generating a second shaped signal and determining the combined value using the second shaped signal.
There is thus still a need for an improved or alternative solution to solve the conflicting requirements encountered in photon counting x-ray detectors.